Frequently items such as electrical instruments are refrigerated by immersion in cryogenic fluids. In such cases it is customary to mount the instrument on a probe which extends into a dewar vessel so that the instrument is located at the bottom of the vessel. The vessel is then filled with a cryogen which is used for refrigeration as it boils off. When the cryogen has boiled off to the point where it no longer covers the instrument, however, the vessel must be refilled in order to provide more refrigeration and still cover the instrument. When the instrument is used in the field, as is often the case, this usually requires that the vessel be removed from use and transported to a convenient filling location. Hence, a need has arisen for an improved dewar vessel for making better use of the contained cryogen and lengthening its "holding time" -- the time during which a vessel of a given size can be used without requiring refilling.
One method of conserving the refrigeration value of a dewar is to increase the thermal conductivity of a selected portion of the dewar's neck tube. One such method, for example, is described in my U.S. Pat. No. 3,705,498. In that patent, the vessel's boil-off gases are directed through a relatively lengthy restricted area of the vessel's exhaust conduit. This increases the velocity of the boil-off gases so that their refrigeration value is more readily passed to a surrounding high-thermal-conductivity collar. In some cases, however, it is desireable to reduce the vessel's temperature below that of its cryogen's normal boiling point. This is usually accomplished by pumping on the cryogen to reduce its pressure and, therefore, its boiling point. In such cases the structure of U.S. Pat. No. 3,705,498 is not entirely satisfactory. Among other reasons this is because the restriction means reduces the cross-sectional area to less than that which is desired to maintain an adequate pumping flow. Hence, the desired lower pressures and temperatures are difficult to achieve. In one of the preferred embodiments about to be described, however, these drawbacks are overcome by arranging one or more porous plates across the vessel's exhaust conduit. These plates do not hinder pumping flow but nevertheless capture the refrigeration from the vessel's boil-off gases and use it to intercept the heat that would otherwise tend to enter the vessel.
Often, such as in the case of Maser dewars for example, the vessels are tipped to an angle of 45.degree. or so with the horizontal so that a quantity of the vessel's cryogen is tipped away from the refrigerated instrument; and, therefore, not available to cover it. Hence, the vessel must be refilled while much of its cryogen remains unused. A preferred embodiment about to be described, however, includes structure for further extending the dewar's holding time when it is used to refrigerate an instrument housed in a vessel that is inclined with the horizontal. This is accomplished by providing a sheet-like retaining collar that extends from the vessel's interior wall toward the instrument so that the collar prevents the cryogen from moving up the interior wall when the vessel is tilted. In this manner a lesser amount of cryogen is required to cover the instrument so that it does not have to be refilled so often.